The present invention relates to a joining and bonding method and an apparatus for joining two members by using a DC current and/or a pulsated current and also to a unit of members joined by the same method, and in particular, to a joining method and an apparatus and also a unit of members joined by the same method, in which a plurality of members to be joined are placed in such a way that surfaces to be joined abut one another and a pulsated current and/or a DC current, in other words, at least either one of the pulsated current or the DC current is applied thereto so that the surfaces to be joined of these members are first joined temporarily, by using a technology such as pulse-excited sintering, which includes spark plasma sintering process (SPS), or a technology of hot-press joining, and thereafter heat-treating said temporarily joined to provide a permanent joint.
Various conventional methods have been used practically for joining a plurality of members desired. There are some well known methods including; (1) welding, in which areas to be joined are heated with a burner, an arc discharge or the like, and a weld-assisting material (i.e. a welding rod) having the same or similar material properties to the members to be joined is melted to be deposited on the members, thereby accomplishing joining; (2) brazing, which unlike welding uses a brazing filler metal (an alloy of Cu, Ag or the like, or an amorphous metal) consisting of a material different from members to be joined, which is melted between the joining surfaces of the members in an inactive atmosphere such as a vacuum atmosphere, nitrogen or argon atmosphere or the like, whereby those members are joined by the medium of said brazing filler metal; and (3) spot welding, in which relatively thin metal sheets are sandwiched between a pair of electrodes and electricity is applied therethrough, to cause welding.
Although the welding operation as described above in (1) includes a process for injecting an inactive gas such as nitrogen or argon to a heated portion to prevent it from being oxidized, since in this method a welding rod is used as the weld-assisting material to be deposited on the member (i.e. to form a padding) along an outer periphery of the joining surfaces, thereby allowing a plurality of members to be joined, such welding is not suitable for use when it is desired to join the entire surfaces of the members. In addition, welding has some defects including, (a) a weld-assisting member must be used; (b) a certain volume of a joining portion of a member must be cut off in advance to enable a welding rod melt to be deposited; (c) a welded portion has to be post-treated or processed, because it is impossible to deposit the melted substance from the welding rod precisely to a desired portion; (d) training is required to learn how to control a heating or welding temperature so that the metal composition of a member to be joined does not change; (e) training is also required to control a cooling temperature and rate so as not to cause a weld crack in the welded portion; (f) a resultant thermal deformation in the areas other than the joined portion and an increase in volume of the joined portion would occur inevitably; (g) the joining of members made of different materials is difficult; and (h) the joining is not applicable to a specific metal such as SUS, due to a change in the properties possibly caused by the welding.
Although brazing as described in (2) is advantageous in that it uses an intervening brazing filler metal placed over the entire joining surfaces, and joining may be accomplished at a temperature lower than that for the above described welding, it still involves some defects such as (i) a brazing filler metal must be used; (j) a joining strength obtained is no greater than the strength of the brazing filler metal; (k) there is an overflow of brazing filler metal upon brazing and thus finish processing must be carried out on the joined portion; (l) it is impossible to join the members in the desired areas precisely due to the overflow of brazing filler metal; (m) a joint uniform in appearance is difficult to attain; (n) strength of the joint will not be uniform especially in the outer periphery, from which the brazing filler metal is likely to flow out; and (o) joining of some metals such as stainless steel (SUS) is not feasible.
Further, although the spot welding described above in (3) can generates thermal energy between the pair of electrodes in a short time and allows narrow areas to be joined in a short time, it still has defects such that (p) only a member of a thin plate-like material can be joined; (q) post-welding distortion is great; (r) an extremely small area can be welded due to the occurrence of partial welding; and (s) post-welding deformation is considerable due to the occurrence of partial welding.
Still further, the above mentioned joining methods are applicable only to the joining process between metal members, and are not suitable for joining a metal member and a non-metal member, or non-metal members.
On the other hand, there has been another joining method suggested that takes advantage of the principle of hot-press joining using a DC current applied to the members to be joined under a predetermined pressure thus accomplishing joining; and there is known a spark plasma sintering process (SPS) which uses a DC pulsated current applied to members to thereby accomplish joining, but a joining method employing SPS technology is disadvantageous in that members to be joined are required to be placed within a die made of graphite and to be joined within said die, which makes the process too complicated. Further there are many restrictions on the shape of the members that can be joined, and, in addition, a sufficiently strong joint can not be obtained. There is known another joining method taking advantage of the principle of the spark plasma sintering process as disclosed in Japanese Patent Application Laid-open Publication No. Hei 11-158514. However, this method also suffers from some problems in that it is applicable only to members having rough surfaces, which entails pre-roughening surfaces for joining, which restrict its use, making it inapplicable, for example, to mirror-surfaces, and the like, and making it difficult to attain a sufficiently strong joint. Still further, even if the conventional hot-press joining process is applied to the joining of bulk members without any modifications, it is difficult to attain a sufficiently strong joint.